Charging of a hole-free thin film phase plate

Abstract

New methods for phase contrast imaging in transmission electron microscopy use the imaging electron beam itself to prepare a hole-free thin film for use as an effective phase plate, in some cases eliminating the need for ex-situ fabrication of a hole and reducing requirements for the precision of the ZPP hardware. The electron optical properties of the ZPP hardware are modified primarily in two ways: by boring a hole using the electron beam; and/or by modifying the electro-optical properties by charging induced by the primary beam. Furthermore a method where the sample is focused by a lens downstream from the ZPP hardware is disclosed. A method for transferring a back focal plane of the objective lens to a selected area aperture plane and any plane conjugated with the back focal plane of the objective lens is also provided.

The invention claimed is: 
     
       1. A method of phase contrast imaging with a transmission electron microscope TEM including a sample holder, an objective lens assembly, one or more intermediate lenses, and an object plane, wherein the method comprises:
 placing a hole-free thin film at a back focal plane of the objective lens assembly, or at a plane conjugate thereto; 
 preparing the hole-free thin film for phase contrast imaging using an electron beam having at least a component thereof focused to a small area on the hole-free thin film at a center of the electron beam; and 
 subsequently imaging a sample in the sample holder by focusing unscattered waves of the electron beam onto a small area on the hole-free thin film, while scattered waves are distributed across the hole-free thin film to impart a phase offset between the scattered and unscattered waves. 
 
     
     
       2. The method of  claim 1  wherein preparing the hole-free thin film comprises using the electron beam to modify electro-optical properties of the hole-free thin film to increase the efficiency of phase transfer by the TEM by charging. 
     
     
       3. The method of  claim 1  wherein the electron beam has an incident electron energy between 100 eV and 1 MeV and the hole-free thin film has a thickness selected to apply a phase delay of π/2+nπ, where n is an integer. 
     
     
       4. The method of  claim 1  wherein preparing the hole-free thin film comprises using the electron beam to modify electro-optical properties of the hole-free thin film to increase the efficiency of phase transfer by the TEM by charging. 
     
     
       5. The method of  claim 1  wherein the transmission electron microscope is a transmission electron microscope, wherein the electron beam is Kohler illumination having a convergence angle of up to about 1 mrad. 
     
     
       6. A method for transmission electron microscope imaging, comprising:
 inserting a hole-free thin film into the transmission electron microscope at a back focal plane of the objective lens or a plane conjugate thereto, wherein said transmission electron microscope comprises at least one lens downstream from the hole-free thin film; 
 applying fields at objective lenses of the transmission electron microscope to focus the electron beam such that unscattered waves from a sample in the transmission electron microscope travel throughout a centre of the hole-free thin film, and scattered waves from the sample pass through the remainder of the hole-free thin film; and 
 adjusting condenser settings of the TEM to improve contrast within the image. 
 
     
     
       7. The method of  claim 6  wherein a lens downstream from the hole-free thin film is used to focus an image of the sample.